Problem 439: Ternary Expression Parser

https://leetcode.com/problems/ternary-expression-parser/#/description

思路

• 分析的时候从右向左，每次只要遇到 ?就停下来。

• 用 stack 来维护第一个值和第二个值。

public class Solution {
    public String parseTernary(String expression) {
        if (expression == null || expression.length() == 0) return null;

        Stack<Character> stack = new Stack<Character>();
        for (int i = expression.length() - 1; i >= 0; i--) {
            char c = expression.charAt(i);
            if (!stack.isEmpty() && stack.peek() == '?') {
                stack.pop(); // pop '?'
                char firstValue = stack.pop();
                stack.pop(); // pop ':'
                char secondValue = stack.pop();

                if (c == 'T') {
                    stack.push(firstValue);
                } else {
                    stack.push(secondValue);
                }
            } else {
                stack.push(c);
            }
        }

        return String.valueOf(stack.peek());
    }
}